X-radiation is a powerful and commonly used tool in modern society. Specifically, x-radiation is commonly used as a diagnostic tool in medical and industrial (non-destructive testing) fields. However, exposure to radiation can be damaging to human health. In order to protect users, patients and x-ray technicians, steps have been taken to limit their exposure to x-rays. For example, flexible x-radiation attenuating materials worn as protective aprons have been created in order to protect patients and technicians alike. In addition, x-radiation equipment has been designed to incorporate with shielding properties to reduce x-radiation exposure from the source generator.
In the field of x-ray equipment, shielding is required or mandated to reduce leakage of stray radiation to below specified maximum levels. Most shielded components of such equipment, such as x-ray tube housings, intensifier housings, collimators and filter devices, typically use a structural outer layer of metal for mechanical strength. This structural component is commonly machined, cast or forged aluminum, brass or steel. Since the aluminum, brass or steel is effectively radiolucent, the structural component is subsequently lined with a second layer of a material, such as lead, for radiation shielding. The lead shielding is typically held in place with an adhesive or by mechanical means, forming a multi-layer final structure with one layer providing strength and structure and the second layer providing x-radiation shielding.
Lead is often the material of choice used for x-radiation shielding in medical and industrial x-ray equipment because it is low in cost and readily available. The use of lead, however, poses significant manufacturing challenges as well as health and environmental hazards. There are two major disadvantages of using lead for radiation shielding: toxicity and the heavy weight of the material. The toxicity of lead has been shown to pose significant health risks to humans. Those adversely affected can include both those involved in processing and those using shielding materials and equipment. The environmental impact resulting from the disposal of products containing lead is also well established and a serious modern-day concern.
In order to limit lead exposure to humans, many industrialized nations regulate industries that use lead, including x-ray equipment manufacturers. In an effort to control, reduce or eliminate the use of lead, many industrialized nations have eliminated the manufacture and use of lead in products such as gasoline and paint. More recently, there has been a general determination to minimize the exposure of workers in plants which now use lead and, more importantly, to shield the general public from the adverse effects from lead in products and equipment and from the toxic waste resulting from the ultimate disposal of lead-containing products at the end of their useful life. The toxicity hazard can result from direct exposure to lead itself or indirectly: from exposure to an extractable source through groundwater leachate from land-fills (Ref. US EPA Toxicity Leachate Characteristic Procedures, “TCLP”, under US RCRA legislation); from solid residues; or from gaseous emissions from waste incineration. The determination to eliminate lead and certain other toxic materials from all electrical equipment, including x-ray equipment, has been established in a European toxic waste elimination directive known as “W.E.E.E.” (Waste Electrical and Electronic Equipment Initiative, 2000/C365/E13) (Jul. 28, 2000).
Not only is lead toxic, but also it is a heavy material that can add significant weight to components such as those required in x-ray equipment. As discussed previously, conventional x-ray equipment contains a lead liner to shield against x-radiation. The excessive weight of lead is especially troublesome because of the mass of lead shielding required to meet mandated radiation leakage standards. Thus, the mass of lead used is a significant proportion of the overall weight of x-ray equipment. Existing manufacturing techniques, which typically involve lining of a separate cast or machined metal structural housing with elemental lead, significantly increase the weight of such equipment. Due to the use of lead, current shielded components are relatively inefficient in terms of mass. They are heavy and complex because the structural metal housing provides insignificant radiation attenuation while the lead shielding, having poor mechanical strength, cannot provide a structural function.
Heavy weight is a significant disadvantage for certain types of x-radiation equipment including portable equipment, such as “C-arm” x-ray diagnostic machines, and for equipment whose shielding components are moving or rotating, such as CT tube housings. In CT tube housings, rotational speeds are limited by inertial forces, which are in turn, dependent on their mass. In these applications, for rotational balance, overall shielding mass is further increased by the need for counterweights to preserve static balance. Lighter shielding components allow lower counterweight mass, which can further result in smaller and lighter supporting structures. Therefore, lowering the weight of the shielded structure can have an overall beneficial effect on the size, weight, cost and portability of x-ray units. Reduced mass can also permit higher rotational speeds of moving parts limited by inertial forces. In CT imaging, for example, reduced mass of the moving parts, especially of the tube head and housing, could permit faster revolution speed, which would lower image acquisition time and/or improve image definition.
While providing adequate shielding at reduced weight is important, replacement components must still fit existing precise equipment designs and keep overall unit size to a minimum. To keep the volume of the structure small, components having high attenuation and also high density are often preferable.
Although mandated radiation leakage testing is usually performed at the worst-case conditions of the peak applied voltage of the machine (typically 70 kVp to 80 kVp for dental x-ray units; 120 kVp to 150 kVp for medical x-ray units; 140 kVp to 160 kVp for CT tube housings; and up to and above 200 kVp for industrial units) shielding must be effective along the entire range of beam energies emanating from the x-ray unit. Shielding components on the receiving end of the radiation, such as intensifier housings, are tested using radiation from the highest voltage from the direct source beam even though they receive only a degraded, filtered and scattered spectrum of radiation in actual practice. Effective substitutes for lead must, therefore, shield radiation not only at the peak voltage of the machine but also along the entire effective range of beam energies and spectra.
Several attempts have been made to create materials that provide acceptable shielding properties but which are lightweight, lead-free or both. For instance, flexible x-ray shielding materials have been available for many years and have been discussed by Yaffe, et al. (Health Physics, Vol. 60, No. 5). Yaffe discussed combining metals with flexible elastomers in order to produce lighter weight radiation protection aprons than those made from similar lead-powder filled rubbers or polymers. These compositions, however, are limited to flexible materials and do not anticipate incorporation of complementary metals into resins in order to create lightweight and rigid, lead-free integral radiation shielding structures.
Recently, there have been attempts to replace only the lead shielding lining of x-ray equipment with lead-free polymer compositions using a single attenuating element. U.S. Pat. No. 4,157,476 to O'Conner describes a shielding material for a dental x-ray tube consisting of a shielding liner composed of barium sulfate filled polymers. This lead sheeting replacement is contained in and acts as a liner in a conventional structural metal housing. The polymer structure in O'Conner simply replaces the lead, while still requiring a layered structure with separate housing as in conventional equipment. Furthermore, the barium sulfate filled resins are implicitly proposed only for shielding dental tube heads which operate at low kVp (below 80 kVp) where barium is an effective x-radiation absorber. Such a shield would be highly ineffective in terms of mass per unit area relative to lead at the higher kVs found in most medical x-ray units. Although barium in elemental form, is more attenuation efficient than lead per unit mass up to about 100 kVp, in actual practice, one would require higher mass for equivalent attenuation. Higher mass would be required because barium is not available in unreactive elemental form, or in useful high concentration alloy form. Barium sulfate, the only available non-toxic barium salt, contains 41% deadweight of radiolucent sulfate, and has low density which prevents high concentration by weight in compounding. Furthermore, the resulting very low composition density would create much thicker shielding liners, on the order of several times thicker than lead for equivalent shielding.
More recently, there have been several additional attempts, both using lead and lead-free formulations, to combine structural and shielding functions in a monolithic polymeric composition. However, these only teach the use of one element for attenuation. For example, U.S. Pat. No. 5,304,792 to Verat describes an x-ray image intensifier tube casing with the outside structural component made from a molded thermoplastic resin loaded with a shielding material such as lead oxide. This technology is specific to intensifier tube housings and uses a single metal or metal oxide (preferably PbO) in an injection moldable thermoplastic resin.
Lead-free filled polymer shielding compositions using only a single attenuating element are also described in U.S. Pat. No. 6,048,379 to Bray et al., which teaches the use of tungsten as the attenuating element in a binder. Bray teaches the use of such material as a lead replacement for use in traditional lead applications, such as projectiles, where density equivalents is desired. Bray also claims tungsten powder in a broad variety of resins formed into articles used for radiation shielding, including housings. Single element-based attenuators such as tungsten have a lower shielding efficiency relative (per unit mass) to lead, or conversely require greater elemental mass than lead for equal shielding at most normal medical beam energies, which typically range from 50 kVp to 150 kVp. Single element based attenuators have a lower shielding efficiency due to a number of factors. In the case of tungsten, with the use of energy beams up to 120 kVp, a significant portion of the beam energy spectrum, including the typical emission spike from conventional tungsten-based anodes, falls in the 55 keV to 69 KeV “K-edge” tungsten window causing poor attenuation. At elevated beam energies, such as 120 kVp to 150 kVp, or at lower kVp but with high beam filtration, the attenuation coefficient of tungsten is simply well below that of lead, both overall, and for the greater part of the beam spectrum. Therefore, the tungsten/resin compositions of Bray do not anticipate and cannot produce shielding or complete monolithic shielded components lighter in weight than elemental lead when used for radiation protection.
Single element-based attenuating compositions, such as those using barium or tungsten, also exhibit a large variability in their shielding efficiency (per unit mass), along the radiation spectrum, compared to lead. None of the above-cited references addresses this issue, nor do cited test data reflect the existence of this shielding factor variability with beam energy and spectra. In addition, a frequent commercial requirement of such compositions and their components is flame retardancy without the use of toxic or hazardous flame retardants. While the use of such flame retardant agents in polymer compositions is known in the art, their introduction to highly filled, dense shielding compositions requires volumetric space. The use of flame retardant agents displaces and reduces the maximum permissible filler loading of attenuating elements, thereby reducing composition density and adding slightly to the required mass of composition for equivalent shielding in an integral structure.
While there have been several attempts at creating lead-free x-ray shielding materials, there remains a need for lightweight and rigid, lead-free, integral, monolithic radiation shielding structures. There is a further need for such x-radiation shielding structures that can shield radiation over a wide range of energies between 50 kVp and 150 kVp, and even up to 200 kVp. The present invention solves the afore-mentioned problems and provides complex molded x-radiation shielding components with varying densities that can be formulated to provide structural strength with x-radiation shielding.